Keyboards are designed for the input of text and characters and also to control the operation of a computer.

A 104-key PC US English QWERTY keyboard layoutPhysically, computer keyboards are an arrangement of rectangular or near-rectangular buttons, or "keys". Keyboards typically have characters engraved or printed on the keys; in most cases, each press of a key corresponds to a single written symbol. However, to produce some symbols requires pressing and holding several keys simultaneously or in sequence; other keys do not produce any symbol, but instead affect the operation of the computer or the keyboard itself. See input method editor.

Roughly 50% of all keyboard keys produce letters, numbers or signs (characters). Other keys can produce actions when pressed, and other actions are available by the simultaneous pressing of more than one action key.

DesignsThere exist a large number of different arrangements of symbols on keys. These different keyboard layouts arise because different people need easy access to different symbols; typically, this is because they are writing in different languages, but specialized keyboard layouts for mathematics, accounting, and computer programming also exist.

The number of keys on a keyboard varies from the original standard of 101 keys to the 104-key windows keyboards and all the way up to 130 keys or more, with many of the additional keys being symbol-less programmable keys that can simulate multiple such as starting a web browser or e-mail client. There also were "Internet keyboards," sold in America in the late 1990s, that replaced the function keys with pre-programmed internet shortcuts. Pressing the shortcut keys would launch a browser to go to that website.

Connection typesThere are several different ways of connecting a keyboard which have evolved over the years. These include the standard AT (DIN-5) connector commonly found on pre-80486 motherboards, which was eventually replaced by the now common PS/2 or USB connection. Prior to the iMac line of systems, Apple Computer used ADB, a proprietary system, for its keyboard connector.

AlternativesA standard keyboard is physically quite large, as each key must remain large enough to be easily pressed by fingers. Other types of keyboards have been proposed for small portable equipment where a standard keyboard is too large. One way to reduce the number of keys is to use chording, i.e. pressing several keys simultaneously. For example, the GKOS keyboard has been designed for small wireless devices. Other two-handed alternatives more akin to a game controller, such as the AlphaGrip, are also used as a way to input data and text. A relatively new type of keyboard, the I-Tech Virtual Laser Keyboard, works by projecting an image of a full size keyboard onto a surface. Sensors in the projection unit identify which key is being "pressed" and relay the signals to a computer or PDA.

Usage

Microsoft On-Screen Keyboard is software designed for users with limited mobility.In normal usage, the keyboard is used to type text into word processor, text editor, or any other textbox.

In modern computers the interpretation of keypresses is generally left to the software. Modern keyboards distinguish each physical key from every other and report all keypresses and releases to the controlling software. This flexibility is not often taken advantage of and it usually does not matter, for example, whether the left or right shift key is held down in conjunction with another character, even though they are coded as completely separate keys.

CommandsA keyboard is also used to type commands in a computer. One famous example on the PC is the Ctrl+Alt+Del combination. With current versions of Windows, this brings up a menu-window including options for handling currently-running applications and shutting down the computer, amongst other things. Under Linux, MS-DOS and some older versions of Windows, Ctrl+Alt+Del performs either a 'cold' or 'warm' reboot.

GamesA keyboard is one of the primary methods of control in computer games. For instance, the arrow keys or a group of letters resembling the pattern of the arrow keys, like WASD, can be used for movement of a game character. In many games, keys can be configured to the user's preferences. Alphabet keys are also sometimes used to perform actions starting with that letter. (e.g. pressing e to eat in NetHack). Keyboards are less than ideal when many keys are to be pressed at once, as the simple keyboard circuitry means that only a certain number of keys will register at one time. A common side effect of this is called "phantom key blocking." Due to the circuit design on older keyboards, pressing three keys simultaneously sometimes resulted in a 4th keypress being registered. Modern keyboards prevent this from happening by blocking the 3rd key in certain key combinations, but while this prevents phantom input, it also means that when two keys are depressed simultaneously, many of the other keys on the keyboard will not respond until one of the two depressed keys is lifted. Better keyboards are designed so that this happens infrequently in office programs, but it remains a problem in games even on expensive keyboards, due to wildly different and/or configurable key/command layouts in different games.

How it worksThe following briefly describes a "dome-switch" keyboard (sometimes incorrectly referred to as a membrane keyboard), the most common type in use today:

When a key is pressed, it pushes down on a rubber dome sitting beneath the key. A conductive contact on the underside of the dome touches (and hence connects) a pair of conductive lines on the circuit below. This bridges the gap between them and allows current to flow (i.e. the circuit goes from open to closed), changing the signal strength. A scanning signal is emitted by the chip along the pairs of lines to all the keys. When the signal in one pair becomes different, the chip generates a "make code" corresponding to the key connected to that pair of lines. The code generated is sent to the computer either via a keyboard cable (using on-off electrical pulses to represent bits) or over a wireless connection. A chip inside the computer receives the signal bits and decodes them into the appropriate keypress. The computer then decides what to do on the basis of the key pressed (e.g. display a character on the screen, or perform some action). Other types of keyboards function in a similar manner, the main differences being how the individual key-switches work. For more on this subject refer to the article on keyboard technology.